The replication dependent histones are expressed predominantly during S-phase. Inhibition of DNA replication by hydroxyurea or UV induced checkpoint pathways result in a reduction in histone transcription and rapid degradation of cytoplasmic histone mRNAs. Recently, the Marzluff lab has demonstrated that histone mRNA degradation requires a post-transcriptional modification that results in the addition of a oligo(U) tail to the 3'end of the histone mRNA. The oligo(U) tail ranges in size from 8-15 nts in length and its addition is required for histone mRNA degradation. Furthermore, our findings indicate that a non-canonical poly(A) polymerase. Terminal Uridyl Transferase (TUTase), is responsible for the oligouridylation of the histone mRNAs. The overall goals of the proposal are three fold. First, I will identify the TUTase isoform(s) responsible for histone mRNA oligouridylation and define the biochemical requirements for its enzymatic activity. Second, I will elucidate the mechanism(s) by which TUTase is recruited to histone mRNAs following inhibition of DNA replication and I will determine if proteins previously reported to be involved in histone mRNA degradation associate with and affect TUTase activity on histone mRNAs. I will also determine if initial oligouridylation of histone mRNA occurs on translationally active polysomes and investigate if TUTase remains associated with the mRNA as it is being degraded. Prior research has demonstrated that over or under expression of histones during S-phase contributes to chromosome loss and DNA damage in yeast and mammals, respectively. Of further importance is the mounting data that post-transcriptional mechanisms are a major determinant of gene expression. As a regulated class of messages, histone mRNAs present us a unique opportunity to identify and understand how the cell cycle regulates mRNA degradation, both as a normal set of processes, and as part of a response to checkpoint pathways important for genome stability.